It is well known to provide a spark plug having first and second electrodes to initiate combustion in a cylinder of an engine by causing an electrical discharge or spark to pass from the first to the second electrode.
It is a problem with such an arrangement that the spark plug takes up space in the cylinder head thereby limiting the size and positioning of the valves used to control the flow of gas into and out of the combustion chamber.
In order to overcome this problem, it has been proposed in European Patent Application 0898058 to combine a spark plug with one of the poppet valves controlling the flow of gas into a combustion chamber of the engine. Although this arrangement eliminates the disadvantage of a conventional spark plug ignited engine by enabling the poppet valves of the engine to be of a larger size and be positioned in a less limited manner, it has several disadvantages.
Firstly, the point of ignition is, as with a conventional spark plug ignited engine, located at a single point in the combustion chamber and so complex inlet and combustion chamber design is required in order to ensure that the mixture to be ignited is positioned at the position where the spark will be generated at the precise time the spark is produced. Secondly, because only a single spark is produced, the time taken for the flame front to propagate throughout the combustion chamber is relatively lengthy and so sufficient time has to be allowed for the combustion process to occur to a satisfactory degree while the piston of the engine is still within an a small range of crank rotation representing an optimum position after top dead center where combustion will produce the maximum torque. This means that the timing of the spark has to occur sufficiently before top dead center for combustion to be virtually complete while the piston is still within the optimum range after top dead center. The position the spark occurs before top dead center is known as the ignition advance angle of the engine and in general terms this ignition advance angle must be increased as the speed of the engine is increased due to the reduction in time available for combustion to occur. However, it is known that the use of large ignition advance angles tend to increase the susceptibility of an engine to knock and this is often a limitation to the maximum running speed of an engine.
In addition, the longer the period taken for combustion to occur the longer the time available for heat to transfer into the engine thereby reducing the thermal efficiency of the engine.
It is an object of this description to provide an improved internal combustion engine.
According to a first aspect of the description, there is provided an internal combustion engine having a cylinder block defining at least one cylinder, a piston slideably supported in each cylinder, a cylinder head defining in combination with each cylinder and piston a respective combustion chamber, each combustion chamber having at least two poppet valves to selectively allow gas to flow into and out of the respective combustion chamber, each poppet valve comprising a valve stem to slideably support the poppet valve in the cylinder head of the engine and a valve head to selectively open and close a gas flow path through a port formed in the cylinder head, at least one of the poppet valves has a portion forming a primary electrode cooperating in use with at least one secondary electrode not formed as part of the poppet valve to produce a number of electrode pairs wherein an electrical discharge is selectively caused to flow during operation of the engine between each of the electrode pairs so as to initiate combustion in the respective combustion chamber of the engine.
There may be several secondary electrodes each forming in combination with the primary electrode a respective one of the number of electrode pairs.
The primary electrode may have a number of discharge tips each forming in combination with the at least one secondary electrode a number of electrode pairs. That is to say, there may be one secondary electrode and several discharge tips.
Each discharge tip may form in combination with a respective secondary electrode one of the number of electrode pairs.
The at least one secondary electrode may be formed as an integral part of the cylinder head located adjacent the primary electrode.
Each secondary electrode may be an integrally formed projection.
Alternatively, the at least one secondary electrode may be fastened to the cylinder head so as to provide an electrical connection therebetween at a position adjacent the primary electrode.
As yet a further alternative, the at least one secondary electrode may be formed on a fuel injector nozzle located within the combustion chamber of the engine.
Each combustion chamber may have two or more poppet valves each having a primary electrode forming in combination with at least one secondary electrode a number of electrode pairs.
All of the poppet valves of each cylinder may have an electrode forming part of at least one electrode pair.
The ignition timing of each primary electrode may be independently controlled.
The ignition timing of each primary electrode may be independently controlled based upon the operating conditions of the engine so as to improve combustion in the respective combustion chamber.
Each poppet valve may be made from an electrically conductive material and may have a head portion forming the primary electrode and the poppet valve may be slideably mounted in the cylinder head so as to be electrically insulated therefrom.
Alternatively, each poppet valve may be made from an electrically insulating material and may have an electrically conductive core portion forming the primary electrode.
As yet another alternative, each poppet valve may be a hollow electrically conductive component defining an internal cavity used to house an internal member made from an insulating material, the internal member having an electrically conductive core portion forming the primary electrode.
According to a second aspect of the description, there is provided a poppet valve for use in an internal combustion engine in accordance with said first aspect of the description.